[swift-evolution] [Pitch] Renaming sizeof, sizeofValue, strideof, strideofValue

[John McCall]

> On Jun 2, 2016, at 8:48 AM, Matthew Johnson via swift-evolution <swift-evolution at swift.org> wrote:
>> On Jun 2, 2016, at 10:38 AM, Xiaodi Wu <xiaodi.wu at gmail.com <mailto:xiaodi.wu at gmail.com>> wrote:
>> 
>> Well, as I understand it, it's not actually possible to write your own type(of:), so we're going from a "magic" property to a "magic" function at least for now.
> 
> No, but you *can* write `func foo<T>(_ t: T)` which accepts any value (you *cannot* write a property that is available for all properties - that would require the ability to write `extension Any`.  This is the distinction I am making.  Of course the implementation is compiler magic no matter how we express it syntactically.  But we can make it *appear* just like it might if the implementation *wasn’t* compiler magic.  That makes it fit into the language better IMO and was the biggest motivator for changing `dynamicType`.
> 
>> 
>> I'm most alarmed that one implication of the MemoryLayout proposal is loss of the `ofValue` family of functions. These functions don't fit with the design: imagine, what is `MemoryLayout<Double>.size(ofValue: Float(42))`? But the response seems to be that these functions don't seem necessary at all and should be removed. "I don't see a use for it" is an insufficient justification for a feature removal. Looking to other languages, C# has sizeof as a static property but tellingly offers the equivalent of sizeofValue (well, strideofValue) as a function in a different module. Essentially every other C-family language that exposes pointers to the user offers both of and ofValue equivalents. The question is, how does a user with existing code using sizeofValue() migrate to Swift 3? I do not see a viable answer with the MemoryLayout design.
> 
> Going with MemoryLayout *does not* mean we would have to give up the value functions if we don’t want to:
> 
> struct MemoryLayout<T> {
>     init() {}
>     init(t: T) { /* throw away the value */ }
>     
>     // we could omit the static properties and require 
>     // writing MemoryLayout<Int>() if we don’t like the duplication
>     static let size: Int
>     static let spacing: Int
>     static let alignment: Int
> 
>     let size: Int
>     let spacing: Int
>     let alignment: Int
> }
> 
> let size = MemoryLayout<Int>.size
> let sizeOfValue = MemoryLayout(42).size

There's no good reason for this type to be generic.  It should be non-generic and require the use of the instance properties.

It's actively harmful for this type to appear to be computed from a value.  The layout is not in any way tied to the dynamic type of the value — for example, it is not the instance layout of the most-derived class or the value layout of the dynamic type of an existential.  Furthermore, saying that it is computed from a value means that attempting to compute it from a type will succeed using the layout of the metatype, which seems like a catastrophic failure of API design.

John.

> 
>> 
>> On Thu, Jun 2, 2016 at 8:03 AM Matthew Johnson <matthew at anandabits.com <mailto:matthew at anandabits.com>> wrote:
>> 
>> 
>> Sent from my iPad
>> 
>> On Jun 2, 2016, at 12:27 AM, Xiaodi Wu via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> 
>>> On Thu, Jun 2, 2016 at 12:24 AM, Patrick Smith <pgwsmith at gmail.com <mailto:pgwsmith at gmail.com>> wrote:
>>> I really like this idea. This IMO is lower level functionality than `type(of:)` (née dynamicType), so I think it makes sense for it to be grouped under its own domain, the MemoryLayout type.
>>> 
>>> Plus MemoryLayout can be extended with new convenience methods.
>>> 
>>> I’m fine with those old methods being removed, but I never use them so! Is it the same as calling type(of:) then using that with MemoryLayout? I imagine they could be fixit’d easily, and that they compile down to the same underlying code.
>>> 
>>> I'm actually souring to the idea. It goes in the diametrically opposite direction from dynamicType. There, something was changed from being property-like to being function-like. Here, Dave's proposal would take something that's a function and turn it into a property. Hmm.
>> 
>> That's not a fair comparison though.  With dynamicType we removed a "magic" property visible on all types, which isn't something you can write and turned it into a function (which is obviously something you can write).  
>> 
>> Dave's MemoryLayout creates a new type to bundle together related items which makes their semantic relationship more clear.  It also receives the type via a generic argument rather than a function argument and makes the properties static.  That is more representative of what is actually happening and could help to prevent confusion.  
>> 
>> If we really need an 'ofValue' option that infers T from a value the properties on MemoryLayout could also be made available as instance properties and it could have an initializer that accepts an instance to T and throws the value away.  However, I'm not at all convinced this is necessary.
>> 
>>>> On 2 Jun 2016, at 3:05 PM, Xiaodi Wu via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>>> 
>>>> 2. Dave A. and others expressed the opinion that these should probably not be global functions; his preference was for:
>>>> 
>>>> ```
>>>> MemoryLayout<T>.size // currently sizeof()
>>>> MemoryLayout<T>.spacing // currently strideof()
>>>> MemoryLayout<T>.alignment // currently alignof()
>>>> ```
>>>> 
>>>> 3. Dave A. proposed that sizeofValue(), strideofValue(), and alignofValue() are better off removed altogether. I don't know if people are going to be happy about this idea.
>>> 
>>> 
>> 
>>> _______________________________________________
>>> swift-evolution mailing list
>>> swift-evolution at swift.org <mailto:swift-evolution at swift.org>
>>> https://lists.swift.org/mailman/listinfo/swift-evolution <https://lists.swift.org/mailman/listinfo/swift-evolution>
> 
> _______________________________________________
> swift-evolution mailing list
> swift-evolution at swift.org
> https://lists.swift.org/mailman/listinfo/swift-evolution

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.swift.org/pipermail/swift-evolution/attachments/20160602/f9c7da99/attachment.html>